Low back disorders, and low back pain (LBP) in particular, remain as the most common and debilitating work-related musculoskeletal disorder. An increasing LBP prevalence with aging, along with an increasing participation of older individuals at work, motivates a better understanding of underlying mechanisms linking aging with LBP. Abnormal mechanics of the spinal column (i.e., higher spinal loads and lower stability) under various work activities/events can eventually result in LBP. Spine biomechanics depend on the physical demands of a task/event (e.g., external loading), passive trunk mechanical characteristics (e.g., stiffness, and damping), and active mechanical neuromuscular response to equilibrium and stability requirements. Despite current knowledge on the age-related degradation of trunk tissues, the overall resultant changes in trunk mechanical behaviors (TMB - both active and passive) are unknown. It is also unknown how changes in TMB with age can influence spine biomechanics under various work activities/events. On the basis of contemporary causal biomechanical theory and our pilot data, two hypotheses were shaped and will be investigated in the present application. Particularly, linking aging with LBP via causal biomechanical theory suggests that: 1) there are age-related changes in TMB and 2) that these changes adversely affect spine biomechanics. Adverse effects on spine biomechanics are increases in spinal loads and decreases in spine stability for a given task/event (e.g., lifting/slp). Changes in TMB that could adversely affect spine biomechanics may include stiffer trunk, reduced damping, increased reflex latency, and decreased reflex response. The aim of present application is to explore such relationships between aging and spine biomechanics by addressing two specific aims: Aim 1: How does TMB change with age? Aim 2: Do such age-related changes in TMB adversely affect spine biomechanics? TMB and spine biomechanics will be evaluated using a novel set of in vivo experimental methods, including passive torso tests (i.e., stress-relaxation) and sudden loading experiments, coupled with finite element modeling and a series of system identification techniques. The emphasis here is on aspects of the biomechanical theory that link a risk factor (i.e., aging) to abnormal spine mechanics, specifically via the influences of the risk factor on TMB and spine mechanics, and which have not yet been explored. Availability of a new set of powerful tools to comprehensively assess TMB and spine biomechanics will enable us to explore these facets of the causal biomechanical theory for the first time. A better understanding of potential causal mechanisms not only contributes to prevention, but also is critical for efficient rehabilitation and safe return-to-wor of workers with occupational LBP. By quantifying age-related alterations in TMB and the resultant effects on spine biomechanics, we expect to establish a foundation from which to develop and implement age-appropriate controls for LBP.